linux-kernel - Re: lockdep and debug objects together are broken?

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Date:	Wed, 21 Jan 2009 08:19:50 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Vegard Nossum <vegard.nossum@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: lockdep and debug objects together are broken?

On Tue, Jan 20, 2009 at 10:11:47PM +0100, Vegard Nossum wrote:
> On Tue, Jan 20, 2009 at 9:55 AM, Nick Piggin <npiggin@...e.de> wrote:
> > Hi,
> >
> > I've had a problem frustrating my testing because lockdep was silently turning
> > itself off... I patched out the code to disable lockdep after the first error,
> > and it started showing up weird errors. kernel/fork.c:990 seemed to be the
> > first to trigger (hard irqs disabled) from a call_usermodehelper call. Later,
> > migration thread was reported to try to unlock rq->lock although it was
> > holding no locks. Then init was reported to return to userspace without
> > releasing an objectdebug hash lock.
> >
> > All that went away and everything seemed to work properly with debug objects
> > configured out.
> >
> > I didn't get too far in trying to debug the problem. But it should be easy
> > enough to reproduce (if not, I can post traces or test patches).
> 
> I just built a kernel with lockdep and debugobjects enabled, and
> everything seemed fine. I think you should post your kernel version,
> config, and the lockdep patch (if needed -- it didn't seem to turn
> itself off here).

Are you sure? Ie. sysrq+D a still works properly? In that case, you
wouldn't need the lockdep patch because it just prevents the feature from being
switched off.

I'll have to dig a bit further, then. The annoying thing is that
lockdep turns itself off at the drop of a hat (and this particular
problem seems to happen without any backtraces), so it invalidates
all your lockdep testing if you don't realise it has turned itself
off.

Is there a way to re-arm lockdep? That would be neat.

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